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There has been another chemical leak at the Orica plant near Newcastle but the local community is not at risk, firefighters say.

The NSW opposition has called on the state government to immediately notify Newcastle residents of the leak.

Fire and Rescue NSW was called to the plant on Kooragang Island around 3.30pm AEDT on Wednesday, following reports of a release of ammonia.

"We have been liaising with plant operators and it appears that a tank containing ammonia has vented some of the chemical due to a build-up in pressure," Acting Deputy Commissioner Mark Brown said.

"The tank has been isolated and the residual pressure is being transferred to another tank so the risk of further releases is very low."

Air samples indicate there is no risk to the public or plant staff.

No evacuations have been required.

Opposition environment spokesman Luke Foley said on Wednesday afternoon his office had in the "last couple of hours" received calls from concerned Stockton residents asking for information.

Mr Foley has called on the government to immediately notify locals of what occurred at the plant, saying it couldn't afford a repeat of its delayed reaction to an August 8 leak of hexavalent chromium from the same ammonium nitrate plant.

It was revealed earlier on Wednesday that Orica is being prosecuted over the earlier leak.

That leak sparked a number of health fears among locals and led to a toughening of state pollution laws.

The Environment Protection Authority (EPA) will now prosecute the company, the NSW Office of Environment and Heritage announced.

"The EPA alleges that Orica breached its Environment Protection Licence in that it failed to operate its ammonia plant in a proper and efficient manner," the authority's Greg Sullivan said in a statement.

"The EPA also alleges that Orica failed to notify the EPA as soon as practicable after becoming aware of the incident and to provide all relevant information to the EPA about the incident.

"Orica has a duty of care to the people of Newcastle and the response from Orica immediately after the incident was not satisfactory."

Orica's ammonia plant at Kooragang Island has been closed under the EPA's orders since August 11.

Orica Australia has been ordered to appear before the NSW Land and Environment Court on February 3.

The August 8 incident led to a tightening of NSW's pollution laws, including new rules forcing companies to report pollution breaches immediately or face a fine of up to $2 million.

The NSW government was also heavily criticised over its handling of the August leak, with Environment Minister Robyn Parker quizzed as to why it took 54 hours for Stockton residents to be informed of the chemical spill.